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Why a Linear Collider Now?

Why a Linear Collider Now?. S. Dawson, BNL October, 2002 Asian, European, and American communities all agree High Energy Linear Collider is next large accelerator WHY???. Where are we going?.

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Why a Linear Collider Now?

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  1. Why a Linear Collider Now? S. Dawson, BNL October, 2002 • Asian, European, and American communities all agree • High Energy Linear Collider is next large accelerator • WHY???

  2. Where are we going? • US high energy community just completed long range planning process • 20 year roadmap for the future • HEPAP subpanel: We recommend that the highest priority of the U.S. program be a high-energy, high-luminosity, electron-positron linear collider, wherever it is built in the world

  3. Linear Collider Basics • Initial design, e+e- at s=500 GeV • Luminosity 1034 cm2/sec 300 fb-1/yr • 80% e- polarization • Energy upgrade to .8-1.2 TeV in future • Physics in  2012

  4. NLC High Power Klystron • The international accelerator community believes that a TeV-scale linear collider can be successfully built JLC Accelerator Test Facility TESLA Superconducting Cavity

  5. Preliminary designs for Linear Colliders TESLA NLC

  6. ????? • What are the big questions we want to answer? • Why do we think we can predict where we want to go? • What do we know now? • What do we expect to learn from the Tevatron and LHC? • What questions will remain unanswered?

  7. What is particle physics? Study of Space, Time, Matter Bagger/Barish report

  8. What is the origin of mass? Do protons decay? Do forces unify at a large scale? Are there more than four dimensions? Why are there 4 forces? The Big Questions? No unification of couplings in SM

  9. What is dark matter? How are particle physics & cosmology connected? What is dark energy? Where did the anti-matter go? Cosmic Connections

  10. Planning for the Future Based on Success of last 20 years…. • Model of electroweak physics verified at .1% level • The problem of mass remains • W and Z bosons discovered at CERN in 1983 • Masses not zero….or even small

  11. Why is Mass a Problem? • Lagrangian for gauge field (spin 1): L=-¼ FF F=A-A • L is invariant under transformation: A (x) A(x)-(x) • Gauge invariance is guiding principle • Mass term for gauge boson ½ m2 AA • Violates gauge invariance • So we understand why photon is massless

  12. Simplest possibility for Origin of Mass is Higgs Boson • Higgs mechanism gives gauge invariant masses for W, Z • Requires physical, scalar particle, H, with unknown mass • Observables predicted in terms of: • MZ=91.1875.0021 GeV • GF=1.16639(1) x 10-5 GeV-2 • =1/137.0359895(61) • Mh • Higgs and top quark enter into quantum corrections,  Mt2, log(Mh)

  13. Precision Measurements sensitive to top quark before it was discovered!

  14. Large number of measurements fit electroweak predictions

  15. Direct measurements of MW, Mt agree well with indirect measurements Prefer Higgs in 100-200 GeV range ASSUMES no new physics Indirect Indications for Light Higgs Mass

  16. Where is the Higgs boson? Precision measurements: • Higgs couplings of fixed • Production rates at LEP, Tevatron, LHC fixed in terms of mass • Direct search limit from LEP: • Higgs contributions to precision measurements calculable G. Mylett, Moriond02

  17. Tantalizingly close….. Direct limit: Mh>114.1 GeV Indirect limit: Mh<193 GeV • New Physics is just around the corner! • Fits assume Standard Model….if Standard Model incorrect, even more exciting new physics….

  18. Higgs mass and scale of new physics correlated….. 130 < Mh < 170 GeV Sensible theory here

  19. at s=2 TeV May discover Higgs if very lucky Requires light Higgs and high luminosity Physics in 2002-2008 Fermilab Tevatron

  20. Upgraded Detectors for RunII CDF Enhanced capabilities for b tagging aid Higgs search D0

  21. pp interactions ats =14 TeV LHC will discover Higgs boson if it exists Sensitive to Mh from 100-1000 GeV Higgs signal in just a few channels Physics circa 2008 CERN Large Hadron Collider (LHC) ATLAS TDR

  22. Discovery isn’t enough…. • Is this a Higgs or something else? • Linear Collider can answer critical questions • Does the Higgs generate mass for the W,Z bosons? • Does the Higgs generate mass for fermions? • Does the Higgs generate its own mass?

  23. How do we know what we’ve found? Measure couplings to fermions & gauge bosons Measure spin/parity Measure self interactions Is it a Higgs?

  24. Coupling Constant Measurements • LHC measures combinations of coupling constants • Typical accuracy, 10-20% • Only some subset of couplings • Assumptions necessary to get couplings L=200 fb-1 Zeppenfeld, hep-ph/0203123

  25. e+e-Zh produces 40,000 Higgs/year Clean initial state gives precision Higgs mass measurement Mh2=s-2sEZ+MZ2 Model independent Higgs branching ratios Linear Collider is Higgs Factory! WWh vertex ZZH vertex

  26. LC: LHC: Direct reconstruction of LC @ 350 Gev Higgs mass measurements Conway, hep-ph/0203206

  27. Dots are experimental error 1-2% measurement Measure ALL Higgs couplings Bands are theory error Larger than experiment Largest error from mb Precision Measurements of Higgs Couplings Battaglia & Desch, hep-ph/0101165

  28. Couplings to fermions very different in SUSY models LC can distinguish SM from SUSY up to MA=600 GeV Higgs measurements test model! Standard Model

  29. Angular correlations of decay products distinguish scalar/pseudoscalar Threshold behavior measures spin Higgs spin/parity in e+e-Zh [20 fb-1 /point] Miller, hep-ph/0102023

  30. ghhh, ghhhh completely predicted by Higgs mass Must measure e+e- Zhh Small rate (.2 fb for Mh=120 GeV), large background Large effects in SUSY Measuring Higgs Self Couplings Lafaye, hep-ph/0002238

  31. Problem with this picture… • Fundamental Higgs is not natural • Quantum corrections to Mh are quadratically divergent Mh22 • So enormous fine-tuning needed to keep Higgs light Mh2\Mh2MW2\Mpl210-32

  32. Solution is Supersymmetry • Quadratic contributions to Higgs mass cancel between scalars and fermions • To make cancellation hold to all orders need symmetry • Bose-Fermi symmetry….supersymmetry

  33. Coupling constants change with energy Coupling constants unifyin supersymmetric models Hint for new physics? Do the forces unify?

  34. New particles in SUSY Theory • Spin ½ quarks  spin 0 squarks • Spin ½ leptons  spin 0 sleptons • Spin 1 gauge bosons spin ½ gauginos • Spin 0 Higgs spin ½ Higgsino Experimentalists dream….many particles to search for! What mass scale? Supersymmetry is broken….no scalar with mass of electron

  35. Supersymmetry • Can we find it? • Can we tell what it is? • Masses of new particles depend on mechanism for breaking Supersymmetry • Couplings of new particles predicted in terms of few parameters • Simplest version has 105 new parameters

  36. Simplifying Assumption: • Assume masses unify at same scale as couplings • Everything specified in terms of scalar/fermion masses at high scale and 3 parameters • Predictive anzatz…..

  37. Discovery of many SUSY particles is straightforward Untangling spectrum is difficult  all particles produced together SUSY mass differences from cascade decays;eg M0 limits extraction of other masses LHC/Tevatron will find SUSY Catania, CMS

  38. SUSY predicts light Higgs SUSY predicts 5 scalars For MA, SUSY Higgs sector looks like SM Can we tell them apart? Higgs BR are different in SUSY Light SUSY consistent with Precision Measurements

  39. Find all the Higgs Bosons Tevatron LHC Carena, hep-ph/9907422

  40. Into the wedge with a LC • s>2MH e+e- H+H-, H0A0 observable to MH=460 GeV at s=1 TeV • s<2MH e+e- H+, H+tb L=1000 fb-1, s=500 GeV, 3 signal for MH 250 GeV

  41. LC can step through Energy Thresholds Run-time Scenario for L=1000 fb-1 • SUSY masses to .2-.5 GeV from sparticle threshold scans • M0/M0 7% (Combine with LHC data) • 445 fb-1 at s=450-500 GeV • 180 fb-1 at s=320-350 GeV (Optimal for Higgs BRs) • Higgs mass and couplings measured, gbbh1.5% • Top mass and width measured, Mt150 MeV Battaglia, hep-ph/0201177

  42. Need to measure masses, couplings Observe SUSY partners, eg Polarization can help separate states Discovery is straightforward e energies measure masses How do we know it’s SUSY? me1 GeV L=50 fb-1 LC Study, hep-ex/0106056

  43. Compare rates at NLO: Lowest order, Super-oblique corrections sensitive to higher scales Masses from endpoints Assume Tests coupling to 1% with 20 fb-1 SUSY Couplings:

  44. What is the universe made of? • Stars and galaxies are only 0.1% • Neutrinos are ~0.1–10% • Electrons and protons are ~5% • Dark Matter ~25% • Dark Energy ~70% H. Murayama

  45. Lightest SUSY particle (LSP) could be dark matter candidate! LSP is weakly interacting, neutral, and stable LSP in range of LC/LHC LC can determine LSP mass; check dark matter predictions Supersymmetry provides understanding of dark matter? LSP is dark matter Mh=115 GeV M1/2 g-2 M0 (GeV) Drees, hep-ph/0210142

  46. Standard Model Needs Top Quark • Top quark completes 3rd generation • Why are there 3 generations, anyways? • Theory inconsistent without top

  47. Top Quark discovery at Fermilab in 1995 Why is Mt(=175 GeV)>>Mb(=5 Gev)?? CDF top event D0 top event

  48. Understanding the Top Quark • Why is ? • Kinematic reconstruction of tt threshold gives pole mass at LC • Compare LHC  2Mt (GeV) Groote , Yakovlov, hep-ph/0012237 QCD effects well understood NNLO ~20% scale uncertainty

  49. tth coupling sensitive to strong dynamics Above tth threshold e+etth Theoretically clean s=700 GeV, L=1000 fb-1 Large scale dependence in tth rate at LHC L=300 fb-1 Top Yukawa coupling tests models Baer, Dawson, Reina, hep-ph/9906419 Juste, Merino, hep-ph/9910301 Reina, Dawson, Orr, Wackeroth Beenacker, hep-ph/0107081

  50. Exciting physics ahead • LHC/Tevatron finds Higgs LC makes precision measurements of couplings to determine underlying model • LHC finds evidence for SUSY, measures mass differences LC untangles spectrum, finds sleptons LC makes precision measurements of couplings and masses • etc

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